From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756438AbZEUB6L (ORCPT ); Wed, 20 May 2009 21:58:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753814AbZEUB6E (ORCPT ); Wed, 20 May 2009 21:58:04 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:60995 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753739AbZEUB6E (ORCPT ); Wed, 20 May 2009 21:58:04 -0400 Message-ID: <4A14B56F.6030109@cn.fujitsu.com> Date: Thu, 21 May 2009 09:59:11 +0800 From: Li Zefan User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Jiaying Zhang CC: Jason Baron , Ingo Molnar , linux-kernel@vger.kernel.org, fweisbec@gmail.com, laijs@cn.fujitsu.com, rostedt@goodmis.org, peterz@infradead.org, mathieu.desnoyers@polymtl.ca, mbligh@google.com, roland@redhat.com, fche@redhat.com Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation References: <20090520073348.GA12316@elte.hu> <20090520154241.GC3081@redhat.com> <5df78e1d0905201849l5e71953bg8444166ae3c8f92a@mail.gmail.com> In-Reply-To: <5df78e1d0905201849l5e71953bg8444166ae3c8f92a@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jiaying Zhang wrote: > Is it possible to convert blktrace to use event tracer? Then in this case we Yes, I'm doing this, see: http://marc.info/?l=linux-kernel&m=124228198011297&w=2 > can pass 'md' as the parameter to trace_block_bio_complete and dereference > md->queue during assignment. > But the problem discussed here exists whether you use plain tracepoints or TRACE_EVENT. Though we can add a new tracepoint named trace_md_bio_complete, this is not the way to solve it. > Jiaying > > On Wed, May 20, 2009 at 8:42 AM, Jason Baron wrote: >> On Wed, May 20, 2009 at 09:33:48AM +0200, Ingo Molnar wrote: >>> hm, this is really a compiler bug in essence - the compiler should >>> delay the construction of arguments into unlikely branches - if the >>> arguments are only used there. >>> >>> We'd basically open-code a clear-cut: >>> >>> trace_block_bio_complete(md->queue, bio); >>> >>> into this form: >>> >>> trace(block_bio_complete, md->queue, bio); >>> >>> .. and this latter form could become moot (and a nuisance) if the >>> compiler is fixed. >>> >>> Have you tried very latest GCC, does it still have this optimization >>> problem? >>> >>> Note that the compiler getting this right would help a _lot_ of >>> other inline functions in the kernel as well. Arguments only used >>> within unlikely() branches are quite common. >>> >>> Ingo >> hi, >> >> I e-mailed the gcc list, where they suggested using a macro, as I've >> done. They also suggested filing an enhancement request for this, which >> I've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207 It seems >> like they agree with the suggestion. >> >> It still might make sense to make this requirement explicit (by adding >> the extra macro), as the tracepoint off case should really be as optimized as >> possible. >> >> thanks, >> >> -Jason >>